Chapter 21 - Buffers and Neutralisation

Buffer solution

• minimises pH changes when small amounts of acid/base are added

• contain weak acid and its conjugate base

Preparation from a weak acid and its salt

• e.g.

  • mixing a solution of ethanoic acid with sodium ethanoate

  • when ethanoic acid is added to water, it partially dissociates

  • when salt is added, it completely dissolves and dissociates into ions

  • salt is the source of the conjugate base

Preparation by partial neutralisation of the weak acid

• adding aqueous solution of an alkali to an excess of the weak acid

• weak acid is partially neutralised by the alkali, forming the conjugate base

• resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid

Removing added acid and alkali

• two reservoirs of the weak acid and its conjugate base

• adding acid

  • [H⁺] increases

  • H⁺ ions react with the conjugate base (A^-)

  • equilibrium shifts to the left, removing most of the H⁺ ions

• adding alkali

  • [OH^-] increases

  • the small concentration of H⁺ ions react with the [OH^-] ions

  • HA dissociates, shifting equilibrium position to the right to restore most of the H⁺ ions

Choosing buffer solutions

• most effective at removing acid or alkali when there are equal concentrations of the weak acid and its conjugate base

• pH of buffer = pK_a of HA

Calculating pH of a buffer

K_a = [H⁺][A^-] / [HA]

Blood plasma optimal pH

7.35 to 7.45

Main buffer of blood

carbonic acid - hydrogencarbonate (H₂CO₃^- / HCO₃^-)

pH changes

• too low

  • acidosis

  • causes fatigue, shortness of breath, shock or death

• too high

  • alkalosis

  • causes muscle spasms, light-headedness, nausea

Carbonic acid - hydrogencarbonate buffer system

• acid added

  • [H⁺] increases

  • H⁺ react with HCO₃^-

  • equilibrium shifts to the left, removing most H⁺ ions

• alkali added

• [OH^-] increases

• the small concentration of H⁺ ions react with the OH^- to make H₂O

• H₂CO₃ dissociates, shifting equilibrium to the right to restore the H⁺ ions

Equivalence point

• The centre of the vertical section of the pH titration curve

• the volume of one solution that exactly reacts with the volume of the other solution

Vertical section

pH increases rapidly on addition of a very small volume of base

End point

the indicator contains equal concentrations of HA and A^- and the colour will be in between

Acid-base indicator

• a weak acid that has a distinctively different colour from its conjugate base

• must choose an indicator that has a colour change in the vertical section of the titration curve

• no suitable indicator for a weak acid weak base titration

Phenolphthalein

• pH 8.3-10

• purple in alkali

• colourless in acid

Methyl orange

• pH 3.1-4.4

• yellow in alkali

• red in acid